Some more information regarding Stroud relation for the aid of conversation:
1. The tests referred to stiff to hard fissured clays and weak rocks.
2. The blow counts were correlated with undrained strenghts from UU triaxial compression tests in 102 mm diameter specimens.
3. The standard splitspoon sampler was used.
4. Generally Stroud highlights the importance of fissuring in clays. According to "Soil Mechanics in Engineering Practise" (3rd edition), the results of Stroud showed a lower value of f1 with increasing clay hardness for the same value of plasticity. This is attributed to the weakening effect of fissures, which is more important in the lab tests.